`#[belongs_to]` doesn't support composite foreign key

[KeenS]

The title is describing problem.

Suppose you have this schema

CREATE TABLE users (
  id SERIAL NOT NULL PRIMARY KEY
  -- ...
);
CREATE TABLE groups (
  id SERIAL NOT NULL PRIMARY KEY
  -- ...
);
CREATE TABLE group_members (
  user_id INTEGER NOT NULL REFERENCES users(id),
  group_id INTEGER NOT NULL REFERENCES groups(id),
 -- ...
  PRIMARY KEY (user_id, group_id)
);

CREATE TABLE member_comments (
  id SERIAL NOT NULL PRIMARY KEY,
  user_id INTEGER NOT NULL REFERENCES users(id),
  group_id INTEGER NOT NULL REFERENCES groups(id)
 -- ...
);

and you'd like to derive Association to member_description

#[derive(Queriable, Ideintfiable)]
#[foreign_key(group_id, user_id)]
struct GroupMember {
  group_id: i32,
  user_id, i32,
}

#[derive(Queriable, Identifiable, Associations)]
#[belongs_to(GroupMember, foreign_key = ???)]
struct MemberComment {
  id: i32,
  group_id: i32,
  user_id: i32,
}

current implementation supports only one foreign key. Are there any chance to allow composite foreign key in belongs_to attr?

Checklist

• I have already looked over the issue tracker for similar issues.

[sgrif]

At the moment we would not be able to support composite foreign keys here without making a breaking change to BelongsTo. At the moment we're not considering anything that would require a major version bump.

[KeenS]

Thank you. I'll use surrogate keys.